I. Field
The following description relates generally to wireless communications, and more particularly to generating a coarse estimate of a symbol boundary with respect to time.
II. Background
In the not too distant past mobile communication devices in general, and mobile telephones in particular, were luxury items only affordable to those with substantial income. Further, these mobile telephones were of substantial size, rendering them inconvenient for extended portability. For example, in contrast to today's mobile telephones (and other mobile communication devices), mobile telephones of the recent past could not be placed into a user's pocket or handbag without causing such user extreme discomfort. In addition to deficiencies associated with mobile telephones, wireless communications networks that provided services for such telephones were unreliable, covered insufficient geographical areas, were associated with inadequate bandwidth, and various other deficiencies.
In contrast to the above-described mobile telephones, mobile telephones and other devices that utilize wireless networks are now commonplace. Today's mobile telephones are extremely portable and inexpensive. For example, a typical modern mobile telephone can easily be placed in a handbag without a carrier thereof noticing the existence of the telephone. Furthermore, wireless service providers often offer sophisticated mobile telephones at little to no cost to persons who subscribe to their wireless service. Numerous towers that transmit and/or relay wireless communications have been constructed over the last several years, thus providing wireless coverage to significant portions of the United States (as well as several other countries). Accordingly, millions (if not billions) of individuals own and utilize mobile telephones.
The aforementioned technological advancements are not limited solely to mobile telephones, as data other than voice data can be received and transmitted by devices equipped with wireless communication hardware and software. For instance, several major metropolitan areas have implemented or are planning to implement citywide wireless networks, thereby enabling devices with wireless capabilities to access a network (e.g., the Internet) and interact with data resident upon such network. Moreover, data can be exchanged between two or more devices by way of a wireless network. Given expected continuing advancement in technology, the number of users, devices, and data types exchanged wirelessly can be expected to continuously increase at a rapid rate.
Communication systems are widely deployed to provide various communication services such as voice, packet data, and so on. These systems may be time, frequency, and/or code division multiple-access systems capable of supporting communication with multiple users simultaneously by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Multiple-Carrier CDMA (MC-CDMA), Wideband CDMA (W-CDMA), High-Speed Downlink Packet Access (HSDPA), Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems.
Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA) are exemplary protocols that are currently utilized in wireless environments to transmit and receive data. OFDM modulates digital information onto an analog carrier electromagnetic signal, and is utilized in an IEEE 802.11a/g WLAN standard, 802.16, and 802.20. An OFDM base band signal (e.g., a subband) is a sum of a number of orthogonal sub-carriers, where each sub-carrier is independently modulated by its own data. Benefits of OFDM over other conventional wireless communication protocols include ease of filtering noise, ability to vary upstream and downstream speeds (which can be accomplished by way of allocating more or fewer carriers for each purpose), ability to mitigate effects of frequency-selective fading, etc.
To effectively employ OFDM as a communications protocol, a boundary between symbols in an OFDM environment often needs to be determined. Such symbols include a plurality of samples as well as a cyclic prefix. The cyclic prefix, for example, can be located at a portion of a symbol first in time, and can include samples that exist within the symbol last in time. Thus, a boundary between symbols that include cyclic prefixes can be determined by locating a cyclic prefix within wireless symbols. A correlating component (e.g., a cross-correlator, an autocorrelator, a delay correlator, . . . ) correlates the cyclic prefix with samples within the symbol substantially similar thereto and determines a correlation in energy therebetween. A peak energy level output by the correlating component is indicative of a boundary of a symbol that can be employed in a wireless environment, and thereafter a fast Fourier transform can be applied to samples in a symbol delivered next in time. If multi-path effects were not an issue and no noise existed upon such channel, the peak energy output by the correlating component could be utilized to precisely determine a boundary between symbols adjacent in time.
Channels, however, are frequently associated with various noise, thus rendering it more difficult to determine location of a peak energy level output by a correlating component. Further, often channels are subject to a multi-path effect, wherein disparate portions of a symbol are delivered over different physical paths (or substantially similar portions of a signal are delivered over disparate physical paths), which can cause delay with respect to a receiver obtaining a plurality of samples. Thus, output of a correlator can produce a heightened flat energy level that does not include a peak corresponding to a boundary between symbols in a wireless network (e.g., OFDM, OFDMA, . . . ). Moreover, when noise accumulates on a channel, accurately determining a boundary between symbols based upon a peak can be difficult. In particular, if there is substantial disparity with respect to location in time of an energy peak output by the correlating component and location of a boundary, errors can result, thereby compromising network performance. In an attempt to alleviate such errors, conventional systems utilize a pre-defined time measurement and utilize such measurement to estimate the aforementioned boundary. In particular, a coarse estimate of a boundary between symbols is obtained by traversing backwards in time from an occurrence of a peak energy level (as output by the correlating component) the pre-defined amount of time. Such a methodology is adequate when a channel is not subject to noise and/or severe multi-path effects. During instances that a channel is associated with substantial noise, this approach can result in error that renders adequate obtainment of a coarse timing estimate between symbols problematic.
In view of at least the above, there exists a need in the art for a system and/or methodology obtaining an improved coarse estimation of a boundary between wireless symbols with respect to time.